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thiosulphate ;   the hydrogen sulphide is then expelled by boiling and the
liquid, first filtered if necessary, tested with barium chloride (sulphate).

3.  Free Alkali.5 grams are dissolved in water and titrated with N/io-
hydrochloric acid in presence of phenolphthalein :   i c.c.  N/io-acid =
0-0040 gram NaOH.

4.  Determination of the Sodium Sulphide  (Battegay's method).
50 grams are dissolved in water to I litre and 50 c.c. of this solution ( = 2-5
grams of substance) neutralised with acetic acid towards phenolphthalein
(disappearance of the red colour) and then titrated with N/2-zinc sulphate
(71-8425 grams ZnS04 + 7H20 per litre) until a drop of the liquid no longer
forms a yellow spot when placed on thick absorbent paper (not ordinary
filter-paper) steeped in concentrated cadmium sulphate solution : i c.c.
N/2-zinc   sulphate   solution = 0-06   gram   Na2S -f gH20 == 0-0195   gram
Na2S.

Commercial crystallised sodium sulphide is usually pure, or almost so, but
the calcined product contains more or less marked proportions of insoluble
residue, thiosulphate, sulphate and free alkali.

SODIUM   SULPHITE

Na2S03

=254

Colourless crystals soluble in 4 parts of cold water to a neutral solution.
It may contain carbonates, bisulphites, thio sulphates, sulphates, chlorides,
and traces of iron and arsenic (tests 1-7). Its value depends on the content
of sulphite (8).

1.  Carbonates.  If sodium carbonate is present, the aqueous solution
has an alkaline reaction towards litmus paper, and addition of lime water
yields a white turbidity or precipitate.

2.  Bisulphites, Thiosulphates.  In presence of bisulphite the solu-
tion is acid.    If thiosulphate is present, sulphur is separated on addition of
hydrochloric acid.

3.  Sulphates.  The solution is boiled with excess of HC1 to expel
sulphurous acid and tested with barium chloride.    It is very difficult to
obtain the sulphite free from traces of sulphate.

4.  Chlorides.  The solution is boiled with excess of nitric acid and
tested with silver nitrate.

5.  Arsenic.  5 grams are evaporated to dryness with pure cone. H2S04
and the residue dissolved in water and tested in the Marsh apparatus or with
hydrogen sulphide.

6.  Iron.  The I : 10 solution is boiled with a few drops of cone. HN03,
then diluted somewhat and tested with ammonium thiocyanate.

7.  Metals, Earths.  In absence of metals and alkaline earths, the
solution   should   show  no  change with ammonium sulphide, ammonia,
ammonium oxalate or sodium phosphate.

8.  Quantitative Determination.  See Sodium Bisulphite.de, sodium thiosulphate